This invention relates to improved redundant operation of counter modules, and to redundant clock drivers and flip-flop modules for counter modules. Clock drive circuits control the frequency of operation in many electronic applications. Reliability of operation of such circuits is of extreme importance in certain of these applications. For example in aerospace applications in which solar or battery DC power is inverted to AC, it is obvious that failure of the clock drive circuit would be catastrophic.
It is expected that such circuits will, in the not too distant future, be used in electric vehicle operation for inverting DC battery power to AC for more efficient power utilization and control. Thus, failure of the clock drive circuit would disable the inverter. If this happened while the vehicle is in high speed traffic on an express highway, the consequences could be disastrous.
Another near future use for such devices is expected to be found in the inversion of solar photovoltaic energy to the equivalent of utility power for supplying the needs of homes in remote or isolated regions. It can be readily appreciated that inverter failure due to clock drive loss would result in AC power failure. Still other situations where reliability is of upmost importance will occur to those skilled in the art.
Reliability is conventionally achieved by use of redundant modules. Any improvement in operation of such redundant arrangements which will provide more reliable operation is therefore of importance. One of the electronic elements frequently used with such clock drive circuits is the flip-flop circuit which is also subject to a high reliability requirement.
Achievement of reliability has been attempted by using modules in a redundant arrangement with appropriate switching to substitute an operative module for a failed module. Such arrangements have the undesirable characteristic that during the module substitution period, passage of the clock drive through the module is interrupted until the operative module takes over from the failed module. That may take a number of clock periods, which in turn may upset the system utilizing the redundant modules. Arrangements which eliminate such interruptions are therefore important.
Even with redundant clock drive circuits and redundant flip-flop modules, there may be a lack of reliable system operation if a counter in the system, which uses both clock pulses from the redundant clock drivers and redundant flip-flop modules, is not reliable. Achievement of counter reliability has been attempted by using redundant arrangement of counter circuits with appropriate switching to substitute an operative module for a failed module. However, unless such an arrangement provides for periodic synchronization of the count of each of the counter modules so that the count is continued in true sequence on substitution of a failed module, the system may fail to operate properly with the substituted module. Consequently, arrangements which provide for periodic cross-resetting of redundant counter modules are important.